Tropical ectotherms are thought to be especially vulnerable to climate change because many live in closed-canopy forests, which provide homogenous thermal landscapes that prevent behavioral buffering of stressfully warm temperatures, and most have narrow thermal tolerance ranges while living close to their upper thermal tolerance limits. Moreover, tropical ectotherms are thought to have decreased capacity for phenotypic plasticity because they have evolved in temporally stable thermal environments. We tested gene expression patterns and phenotypic plasticity of thermal traits in Panamanian slender anoles (Anolis apletophallus) by a) measuring changes in gene expression in response to short-term warming and cooling (two hours) and b) using a mesocosm experiment to measure phenotypic plasticity in response to long-term warming (one month). Many genes were differentially expressed in the brain, liver, and muscle in response to short-term warming, including genes that coded for heat shock proteins, and gene upregulation occurred primarily in response to warm conditions rather than cool conditions. During long-term warming, we found that lizards developed increased voluntary thermal maxima but displayed limited plasticity in thermoregulatory behavior in a laboratory thermal arena. Our results indicate that tropical forest lizards can use gene expression and phenotypic plasticity to respond to shifting environmental temperatures, and that these processes should be considered when predicting the future of tropical ectotherms under a changing climate.